The invention relates to a brake booster, and more particularly, to a brake booster which assures that an increased output can be obtained in response to a force of depression of a small magnitude which is applied to an brake pedal in the event a braking effort of an increased magnitude is required as in an emergency brake.
A brake booster which is arranged to permit an increased output to be obtained even in response to a brake depressing force of a small magnitude as in an emergency brake which requires a braking effort of an increased magnitude is known in the art, as exemplified by Japanese Patent No. 3,070,672 disclosing a brake booster in which in response to an input which exceeds a given value, an output gradient is increased subsequently to increase the output, or a Japanese Laid-Open Patent Application No. 35,474/1998 disclosing a brake booster in which an operation of an brake pedal is electrically detected, and the output is increased in response thereto.
However, drawbacks are pointed out with conventional brake boosters as mentioned above. Specifically, in the brake booster disclosed in ""622 patent, the arrangement is such that the output is increased when the input has exceeded the given value, and accordingly, the output can not be immediately increased in response to a quick depression of the brake pedal.
In the brake booster disclosed in Application No. 35,474/1998, there is a need to provide a solenoid operated valve and a sensor which detects the degree of depression of the brake pedal in an integral manner with the booster, resulting in a complicated arrangement and an increased manufacturing cost of the brake booster.
In view of the foregoing, it is an object of the invention to provide a brake booster which is simple in construction and which is capable of immediately increasing the output in response to a quick depression of a brake pedal.
Specifically, the invention relates to a brake booster including a valve body slidably disposed within a shell, a power piston mounted on the valve body, a constant pressure chamber disposed forwardly of the power piston within the shell, a variable pressure chamber disposed rearwardly of the power piston within the shell and a valve mechanism for controlling a switching between the supply/discharge of a fluid to and from the variable pressure chamber, the valve mechanism comprising a vacuum valve seat formed on the valve body, a tubular member disposed within the valve body to be axially movable relative to the valve body and having an atmosphere valve seat formed toward the rear end thereof, a valve element having a first seat adapted to be seated on the vacuum valve seat and a second seat adapted to be seated on the atmosphere valve seat, and a valve plunger connected to an input shaft and disposed to be axially movable relative to the valve body so that it is driven forward together with the tubular member when at least the input shaft is driven forward. In accordance with the present invention, there is provided engaging means which connects the tubular member and the valve body together to maintain the second seat of the valve element to be removed from the atmosphere valve seat during the time the input shaft is being driven forward when the input shaft is driven forward relative to the valve body through a given stroke, the engaging means all owing the connection between the tubular member and the valve body to be terminated when the input shaft retracts to its inoperative position.
With the described arrangement, when the brake pedal is quickly depressed and the input shaft is driven forward through the given stroke relative to the valve body, the engaging means maintains the second seat of the valve element removed from the atmosphere valve seat, thereby allowing the atmosphere to be rapidly introduced into the variable pressure chamber of the brake booster to increase its output. In this manner, there is provided a brake booster which is simple in construction and which is capable of immediately increasing the output in response to a quick depression of the brake pedal.
Above and other objects, features and advantages of the invention will become apparent from the following description of the embodiments thereof with reference to the attached drawings.